Plant Communities and Soil Moisture Relationships Near Denver, Colorado

Abstract

Plant communities and some causal factors were studied on a stony soil, on pebble mounds on stony soil, and on an adjacent soil derived from the Pierre Shale in an area 14 miles northwest of Denver, Colorado, at approximately 6,000 ft altitude. Average annual precipitation is 15.2 inches. The three dominant perennial species on the soil derived from Pierre Shale were buffalograss, blue grama, and western wheatgrass, all of which are characteristic of the mixed prairie association of the Great Plains. True prairie species characteristic of prairies of the Midwest, big bluestem, little bluestem, Indiangrass, switchgrass, and prairie dropseed, were present in significant amounts on the stony soil; however, most of the other species were montane. Some of the montane species were mountain muhly, sandwort, and beardtongue. More than twice as many species occurred on the stony than on the other two soil conditions. The greater availability (lower tensions) of soil moisture in the stony soil appears to be the primary cause of the larger number of species and generally more mesic flora. Infiltration rates were highest on pebble mounds and stony soil. Although the stony soil contained less zinc, potassium, sodium, and sulfate, plant species from the stony soil contained larger quantities of these nutrients. The higher pH in the shale—derived soil (7.1 as compared to 6.0—6.3) and larger quantities of montmorillonitic clays may have reduced the availability of these nutrients to plants. The presence of vegetation on pebble mounds that indicates either disturbance or droughty conditions is attributed to the activities of the mountain pocket gopher. Species prominent on mounds were cheatgrass, sixweeks fescue, ragweed, and hairy goldaster. Similar soil moisture conditions in mounds and in stony soil indicate that the two soils should have the same kind of vegetation if disturbance is not a factor. Evidence suggests that pocket gophers may be the causal factor for both the mounds and the subclimax vegetation on mounds.